Vacuum roll coated security thin film interference products with overt and/or covert patterned layers

ABSTRACT

A security thread for embedding in or on a sheet such as a document, currency or packaging is disclosed, wherein the thread has a thin film interference structure deposited thereon. The structure may be a Fabry-Perot structure in the form of a single filter, or may be plural filters, spaced side-by-side. The thread appears to have separated interference filters deposited thereon. Preferably the thread is first roll coated with oil using an oil-ablation technique to remove deposited material in situ in the coating process. A magnetic layer may be deposited within, behind or in front of the thin film interference structure.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims the benefit of previously filed,co-pending U.S. Application Ser. No. 60/487,527, filed Jul. 14, 2003, byPhillips et al, entitled: “IMAGED OPTICAL FOILS,” the disclosure ofwhich is incorporated herein.

BACKGROUND OF THE INVENTION

The use of security threads for protecting banknotes, credit cards andother valuable documents is well known. A security thread is a strip ofmaterial placed on the surface of a banknote document or sheet such asbanknote; alternatively a security thread may be serpentined or woveninto the banknote paper (a window type effect) to confer additionalsecurity (authenticity) to the bank note. Typical dimensions of a hotstamp thread are a width of 1-5 mm, a thickness of 3-4 μm; windowedpolyester terephthalate (PET) based threads have a thickness of about0.5 mil or 12.5 microns. By way of example, one of the earliest forms ofsecurity threads consisted of reflective foil transferred by hotstamping to the surface the banknote (GB 2119312 A). This reflectivefoil prevented reproduction of counterfeit banknotes by printingprocesses such as from printing presses, PC printers and copiers.Holograms (EP-A-0624688), holographic features along with thermo chromicfeatures (GB 2347646), opaque coatings having characters and patternsreadable by transmitted light in combination with luminescent substances(U.S. Pat. No. 6,474,695), repeating patterns of magnetic/magneticindicia or metal dots (W02103624), laser etching fine lines and textwith a laser (German “Auslegeschrift” no. 22 05 428) and (WO02101147),printing micro-characters on a metalized transparent plastic with clearacid resistant inks followed by acid etching of the unprinted areas toproduce shiny micro-characters on a transparent base (U.S. Pat. No.4,652,015), bonded nucleic acid molecules so that complementary nucleicacid molecules can bind to the molecules already attached to thedocument (DE 10122836), and optically variable security elements usingliquid crystal material (EP0435029) have all been used to make securitythreads. However, these aforementioned threads either take too much timeto make and or have other associated problems; for example, it is foundthat laser etching takes too long to be cost effective, etching by useof chemicals requires multiple steps and is not considered to beenvironmentally-friendly; holograms can be readily copied, and in manyinstances the features of the threads are not readily seen by eye by theaverage person and machines are required to read them.

A method to pattern a single layer of metal or carbon in a vacuumchamber was advanced in U.S. Pat. No. 4,022,928 by Piwcyzk. Piwcyzk usedvarious methods to apply a perfluoropolyether known as FOMBLIN™ orKrytox™ to a substrate requiring a pattern for a vacuum deposited layer.The perfluoropolyether inhibited the deposition of the depositingmaterial to a web or plastic substrate. Application of this fluid was byspray or vacuum evaporation in combination with a selected removalprocess as with a laser or an electron beam. A printing method was alsodescribed. Printing techniques including relief printing such asletterpress or flexography, planographic printing such as offsetlithography, and gravure, and screen-printing such as silkscreen processprinting were disclosed.

Subsequently, Ronchi in U.S. Pat. No. 4,749,591 incorporated herein byreference, and in PCT application WO 8700208(A1)) advanced this printingprocess by applying the inhibiting oil, FOMBLIN, to a vacuum roll coaterwhere patterning thin films on plastic substrates was desired.

Ronchi in U.S. Pat. No. 4,749,591 only discloses applying a single layerof metal, for example, aluminum as is shown in FIG. 1, deposited as avacuum thin film layer. A demetallized aluminum layer in the case of asecurity thread embedded into a banknote can easily be forged by simplyusing a metallized polyester that is subsequently patterned by one ofthe above methods. In an attempt to copy a security thread having asingle layer manufactured by Ronchi's technique, patterning byphotolithography in combination with a caustic etchant, or by any of theaforementioned processes or even by using a silver pencil to simulatethe security thread could be used. Security threads having multi-layerfilms where one of more of the layers are patterned has not previouslybeen considered. A major impediment to providing several thin filmlayers, was residual oil remaining on the images and on non-patternedareas of the web. This residual oil was detrimental to further thin filmcoating since left over oil would cause “ghosting”; a process wherebythe inhibiting oil is transferred to the back side of plastic film whenroll coating, which in turn causes inhibiting oil to be transferredfurther down the web on the front side. Left over inhibiting oil alsocauses adhesion failures to subsequent thin film layers.

“Ghosting” and the ability to remove residual inhibiting oil is overcomeby this invention. By way of this advance, for the first time, patternedmultilayer optical stacks could be conveniently manufactured in a costeffective way on a security thread, by roll coating. In particular, anew optically variable security thread that had a high patternresolution was made that contained readable text or graphic images wherecovert features such a magnetic signatures could also be incorporated.

It is an object of this invention, to provide a security thread havingoptically variable features such as an optically variable pattern thatcan be seen against a background that is distinguishable from thepattern, or from which the pattern stands out.

It is a further object of this invention, to provide a relativelysimple, inexpensive method of manufacture of a multilayer patternedsecurity thread for use within or upon a web or sheet, for example oncurrency, documents or packaging for providing authentication thereof.

SUMMARY OF THE INVENTION

In accordance with this invention there is provided, a security threadfor embedding within or upon a sheet or document, comprising:

-   -   an elongate substrate having a first side and a second side;    -   an optically variable structure deposited on one of the first        and second side of the elongate substrate, wherein the optically        variable structure comprises a thin film interference structure        which has the appearance of a plurality of separated        interference filters arranged side-by-side and spaced from one        another having visible color shifting properties in the form of        a visible pattern of visually separated distinguishable indicia        against one of a foreground and background of a different color.

This invention provides a security thread providing security to a sheet,document or packaging, wherein thread has a visibly optically variablestructure thereon that is visible from at least one side of the sheet;although the optically variable structure may be a continuous pluralityof layers forming a large Fabry-Perot cavity or interference filter, byproviding a patterned layer in front of the Fabry-Perot cavity, theinterference filter appears to be separate spaced filters. In anotherembodiment plural separated filters are provided which similarly appearas separated optically variable structures.

In accordance with this invention there is provided, a sheet having asecurity thread embedded therein or disposed thereon, the securitythread comprising:

-   -   a substrate having a first side and a second side;    -   a plurality of separated n-layered Fabry-Perot cavities        deposited upon the first side of the substrate, side-by-side,        wherein the Fabry-Perot cavities are spaced from one another,        wherein each n-layered Fabry-Perot cavity is a thin film        interference filter having visible color shifting properties;        said plurality of cavities being arranged along the substrate to        form a visible pattern as a result of the color shifting        properties.

In accordance with another aspect of this invention there is furtherprovided, a security thread for imbedding within or disposing upon asheet, wherein the security thread comprises a plastic web upon which isdeposited layers of thin film color shifting coatings formingside-by-side, spaced apart interference filters, wherein theinterference filters are seen as patterns against a background of adifferent color.

In accordance with one embodiment of this invention, a continuous FabryPerot structure having plural layers defining one or more cavities canbe applied to one side of a web or substrate. On a second side of theweb, a pattern of aluminum or some other material visiblydistinguishable from the Fabry Perot structure can be applied using anoil ablation process. Visually when one views the thread from thepatterned aluminum side, the portions absent aluminum show as opticalvariable regions, and the portions with aluminum present show ascontrasting aluminum regions. Hence in this embodiment, the continuousFabry-Perot structure reveals to the viewer from the patterned side,plural side-by-side spaced FP cavities since the aluminum masks portionsproviding the pattern.

In accordance with yet another aspect of the invention there is provideda method for forming an optically variable device comprising the stepsof:

-   -   patterning a reflective layer on a web substrate having a first        side and a second side using an oil-ablation technique to form a        patterned reflective layer;    -   removing oil residue from the first side of the web and        depositing thin-film layers on the web substrate to form the        optically variable device.

In accordance with another aspect of the invention there is furtherprovided a method of patterning metal comprising the steps of:

-   -   applying a non-wetting oil to selected portions of a first        surface of a web substrate to create an oil pattern;    -   depositing metal on the first surface of the web substrate        wherein the non-wetting oil ablates depositing metal from the        oil pattern; and    -   removing oil residue using a first glow discharge applied to the        first surface of the web and a second glow discharge applied to        a second surface of the web.

In accordance with another aspect of the invention there is provided amachine-readable security device, comprising a web having disposedthereon, a patterned layer of magnetic material sandwiched between twometal layers.

In accordance with the invention a machine-readable security device isalso provided wherein a security thread includes a magnetic materialpatterned thereon using an oil-ablation process.

The use of an oil ablation process is a preferred embodiment, allowingthe coating and removal of a rolled on pattern within a coating chamberto yield a patterned web having a visible patterned interferencestructure. Notwithstanding, it is within the scope of this invention touse of other materials having similar properties to oil, wherein itsremoval is compatible with the coating of subsequent layers in situ.Alternatively, but less preferably, a water soluble polymer coating thatcan later be removed by washing is possible, however temporary coatingsof this sort are not as useful as the application of oil, which can beremoved within the deposition chamber.

This invention circumvents difficulties encountered in wet chemicaletching methods for patterning by providing a novel security thread thatis optically variable either in reflection or transmission with text orother patterns by using an all-dry process, in-line, in a vacuum rollcoater. The human eye can readily see the optical performance of thethread as a color shift as the thread is tilted back and forth. Eitherthe text or pattern is optically variable against a transparent or shinyreflective background, or alternatively the background is opticallyvariable against the text or patterns that can be easily viewed intransmission. Furthermore, the thread and its pattern can bedistinguished from the background of the sheet, which carries it.Alternatively, the thread can be viewed in reflection where the imageryappears to be colored or optically variable using foil or color shiftingink against a reflective background of aluminum or other colored metalsuch as copper, or an optically variable, or a non-optically variablethin film optical stack. Alternatively an embodiment of the inventionprovides the inverse or negative image of the above-mentionedstructures. The optically variable optical stacks can take the formdescribed in U.S. Pat. Nos. 4,705,356; 4,838,648; 5,135,812; 5,214,530;5,278,590; 5,278,590; 6,157,489; 6,241,858; 6,243,204; 6,241,858;6,569,529; and 6,699,313 to Phillips, an inventor of this invention. Inaddition, mica based interference pigments such as TiO₂ or Fe₂O₃ coatedmicas can be used as the color shifting pigments in the color shiftinginks.

Various embodiments of this invention are described and shown indetailed description and accompanying figures. The thread can either bewindowed into the banknote in much the same manner as that found inEuropean patent application EP1258334 A3 in the names of Cunningham, andBrian or can be applied across the surface of the banknote.

Such threads as described in accordance with this invention cannot beaccurately reproduced by way of being photocopied, photographed, orprinted since these technologies do not possess optically variableeffects. Furthermore, in the case of optically variable foil, the opticsof copiers prevents even the face color at normal incidence from beingimaged; since just a black image results as the reflective surface ofthe optically variable thread causes the light to miss the entranceoptics of the copier. In addition, the intricate design of the text,having a resolution down to 60 microns, would prevent any counterfeiterfrom using scissors to simulate this security device.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention will now be described inconjunction with the drawings in which:

FIG. 1 is a diagram illustrating a prior art device wherein a layer ofpatterned aluminum on a PET substrate or web.

FIG. 2 is a cross sectional view showing an embodiment of the inventionwherein interference structures are shown created by layer of patternedaluminum on a PET web covered with a spacer layer of MgF2 capped by a Crlayer, the three layers forming a plurality of optical interferencestructures, side-by-side having windows, therebetween.

FIG. 3 is a cross sectional view of a layered structure with patternedaluminum on one side of a PET web, an optically variable foil coating onthe same side of the PET web, and a different optical coating on theother side of the PET web.

FIG. 3 b is a cross sectional view similar to that of FIG. 3, with theaddition of an embossing of the plastic layer.

FIG. 4 is a diagram of an embodiment illustrating a patterned magneticlayer on a PET web.

FIG. 5 shows an embodiment of the invention wherein a patterned magneticlayer is sandwiched between two layers of aluminum on a PET web.

FIG. 6 shows an embodiment with a patterned aluminum layer on one sideof a PET web and an optically variable foil with a hidden magnetic layeron the same side of the PET web.

FIG. 7 is an illustration showing a patterned aluminum layer depositedon a plastic substrate over-coated with color shifting ink-on one sideof a PET web.

FIG. 8 shows the effect of positive and negative patterned aluminumlayer over-coated with color shifting ink layer on one side of a PETweb.

FIG. 9 is a diagram of an embodiment of the invention wherein apatterned aluminum layer is deposited on one side of a PET web and acolor shifting ink layer is coated on the opposite side of the PET web.

FIG. 10 is a cross sectional view illustrating a patterned aluminumlayer deposited on one side of a PET web over-coated with a protectivelayer of abrasion resistant lacquer and a color shifting ink on theopposite side of the PET.

FIG. 11 shows a patterned aluminum layer over-coated with a Fabry-Perotdesign on one side of a PET web and coated with a second differentFabry-Perot design on the second side of the PET.

FIG. 12 is a cross sectional view of a patterned aluminum layer on oneside of the PET web and a continuous optical structure on the secondside of the PET.

FIG. 13 shows a patterned colored shifting foil and one side of the PETand a printed layer of ink (black or complimentary color to the colorshifting foil) on the second side of the PET.

FIG. 14 is a cross sectional view showing a Fabry-Perot design on a PETweb where the dielectric spacer layer is patterned.

FIG. 15 is a cross sectional view showing a Fabry-Perot design on a PETweb where the absorber layer is patterned.

FIG. 16 is a diagram, which illustrates a simplified coater according toan embodiment of the invention.

FIG. 17 is a diagram which illustrates a cleaning arrangement within thecoating chamber to remove residual “inhibiting oil”.

FIG. 18 is a view of a one dimensional magnetic bar code pattern hiddenwithin the aluminum layers, wherein the Al layers are conveniently shownin this figure to be transparent so that the bar code can be seen.

FIG. 19 shows a two-dimensional magnetic bar code pattern hidden withinthe aluminum layers.

FIG. 20 is a photograph which shows the resolution of a text symbol in apatterned optically variable security thread.

FIG. 21 is a photograph of a patterned optically variable threadembedded into a bank note in a windowed type format.

FIG. 22 is a photograph showing the text of the patterned opticallyvariable thread in a bank note viewed in transmission.

FIG. 23 shows a graphic security label formed from a patterned opticallyvariable foil structure on a pressure sensitive adhesive-releasablelabel paper stock.

DETAILED DESCRIPTION

Referring now to FIG. 2 a layer of patterned aluminum 22 on a PET web 20is shown. This embodiment is not limited to the use of aluminum as areflector material and other reflecting materials, for example anotherreflecting metal could be used instead of aluminum. The PET web formsthe base of the security thread upon which the layers shown aredeposited; however other materials, such as other plastics could be usedin place of PET. The aluminum patterned layer 22 is covered by a spacerlayer 24 of MgF2 deposited over the patterned aluminum and web whichforms windows 28 in regions over the web absent the deposited aluminum;a layer of absorber material 26, such as a thin layer of chromium, isdeposited over the spacer layer 24. An optical interference structure isformed from the reflector/dielectric spacer/absorber (R/D/Ab) stack overthe remaining portions of the patterned metal, but not over the portionsof the web where the Al has been removed; these portions were Al hasbeen removed are referred to as window portions. The opticalinterference structure(s) 21 can be color filters that gives thepatterned metal a particular appearance, or one that gives the patternedmetal a color-shifting, “optically variable” appearance. The aluminum ispatterned by printing an image or pattern onto the plastic web 20 usingthe “inhibiting oil” and then depositing a thin film of aluminum.Although the exact mechanism by which the oil prevents the sticking ofthe vacuum deposit to the substrate may not be entirely understood, theprocess nevertheless works. Various theories have been proposed toexplain this phenomenon. One theory invokes the idea that the heat ofcondensation of the depositing material turns the oil into a gas and ineffect ablates the metal away. Another explanation is that the oilsimply prevents nucleation of the depositing material and hence thearriving material is scattered away.

In the embodiment shown in FIG. 2 the Al—MgF₂—Cr stacks each form aFabry-Perot (“F-P”) absorber-spacer-reflector-type optically variabledevice (“OVD”), which are not formed over the window portions of the webbecause there is an absence of a Fabry-Perot structure in these areas.The plastic web can be clear, tinted, translucent, or opaque, and thematerials chosen for the patterned metal layer and overlying thin filmsare merely exemplary. After coating the thin-film layers, a protectivelayer is optionally applied, such as a thin layer of lacquer, not shown,or a thin (e.g. 0.5 mil) plastic film that is adhered to the OVD usinglaminating adhesives.

FIG. 3 shows a layer structure, similar to that of FIG. 2 on a frontside of a web 30. Patterned aluminum is deposited after using aroll-coater with inhibiting oil to apply a pattern of oil to preventpermanent deposition onto predetermined regions of the web 30 on a“front” side of a PET web. Layers 34 and 36 of MgF₂ and Cr are depositedover the front side to form optical interference structures over the Al;the Al serves as a reflector in the Fabry-Perot structure(s). Anotheroptical structure 39, such as a reflective layer, an optically variable(OV) layer, a magnetic layer, either continuous or patterned, eithersandwiched between layers of aluminum or as a single layer orfluorescent layer is formed on the “back” side of the web. In thisembodiment, a protective layer is optionally applied to both surfaces ofthe OVD. The optical structure may be opaque or semi-transmissive.

In accordance with this invention, in the embodiment where a magneticlayer is sandwiched in between the reflective layer, as is described inU.S. patent application No. 2002/0160194A1 and WO 02090002(A2), in thename of the same inventor, a cover signature is present. A number indigital code or as in a bar code may be present, that is unseen by thenaked eye, and may be fore validating the serial number on the bill orthe denomination for example, $50.00.

In this embodiment, where a magnetic bar code is hidden within thedevice, it exhibits the same reflective properties as aluminum but has amagnetic signature that can be read with appropriate magnetic detectors.The magnetic detection may be just the presence of a magnetic materialas in a magnetic image such as a hidden bar code or a hidden logo, ormay be a signal of digital or analog recorded information.

In accordance with this invention, the thickness of the magnetic layeris preferably between 0.1 and 1.0 microns in thickness. In the priorart, it is known to use a thick layer for example having a thickness of12 to 13 microns. Notwithstanding, this invention can provide a verythin layer that is detectable by providing a continuous layer ofmagnetic metal or metal that is detectable by magnets. Thinner layersare advantageous for use with security threads to be imbedded in or oncurrency so that a large stack of bills do not pile up at an angle whenstacked. Furthermore, providing thinner layers ensures that that overallthickness of the thread remains relatively thin, which is desired.Hence, the thickness of the magnetic layer should be less than 5 micronsand preferably less.

In the simplest case, as shown in FIG. 4 a single layer 42 of magneticmaterial may be deposited onto the plastic web 40, patterned by the oilimaging method, and then formed into a security thread, label or hotstamp image. In more complicated structures, the patterned magneticlayer 52 supported by a substrate 50 is sandwiched between two layers ofaluminum 54 and 56 as is shown in FIG. 5 or is sandwiched between twolayers of aluminum in a Fabry-Perot optical stack shown in detail inFIG. 6 wherein a substrate 60 has deposited thereon a patterned aluminumlayer 62; upon the layer 62 is a Cr layer 63 a dielectric layer 64, anda magnetic layer 66 between two Al layers 65 and 67 upon the dielectriclayer 64. In the instance where only one side of a security thread isvisible, the magnetic layer 66 can be covered by a single layer ofaluminum 67 and need not be sandwiched between two such layers.

Referring now to FIG. 5, a cross-sectional view is shown wherein aplastic substrate 50 has deposited thereon a non-magnetic layer 54 ofaluminum. Of course other non-magnetic materials could be used. Apattern of inhibiting oil is then applied to create a predeterminedbar-code pattern of oil dependent upon the design upon the rollers thatpick up the oil and coat the plastic web 50. The magnetic layer 52 issubsequently deposited and magnetic material only remains where no oilhas been applied during the vacuum coating and cleaning process. A finallayer of non-magnetic material 56 effectively sandwiches the magneticlayer 52 between the two non-magnetic layers.

In an alternative embodiment now shown, one could use an additionallayer over a patterned layer to serve as a leveling layer. This could bedone by evaporating an organic smoothing layer in vacuum and curing bycross linking as taught by Yializis in U.S. Pat. No. 6,706,412. In theembodiment described in accordance with this invention, the depth of thevacuum deposition layer is considerably thinner than the thickness ofthe oil pattern; hence in relation to the oil thickness of about 10,000Angstroms the small bumps of 1000 Angstroms or less for the vacuumdeposited layer would be negligible. Patterning for multilayers couldtake place on unpatterned regions or even on previously patternedregions.

FIG. 6 is a more complex structure than that of FIG. 5, however themethod of manufacture is essentially the same, patterning and depositingsubsequent layers.

In yet another embodiment of the invention the combination of patternedthin film with color shifting ink including but not limited to inkscontaining pearlescent type pigments based on coated mica, SecureShift®colors (registered to Flex Products), optically variable ink (OVI®,registered to SICPA), inks based on diffractive based pigments or liquidcrystal color shifting inks can be present. The pigment may be formed offlat thin film optical structures or may be formed of diffractive flakesas described in U.S. Pat. No. 6,692,830 and PCT patent application WO03011980A1.

Referring now to FIGS. 7 and 8 an embodiment of the invention is shownthat utilizes color-shifting ink. In this instance, the color shiftingink 75 is visible through the holes or windows 76 in the patterned thinfilm 73. In the simplest case, a patterned aluminum 73 is formed on aplastic film 70 such as polyester terephthalate (PET) and the colorshifting ink 75 is coated over the patterned aluminum so that from theopposite side, one sees a color shift with viewing angle through thetext or graphic images or appears as background around reflective textor graphics.

In FIG. 8 is an illustration showing that the background can benon-color shifting and foreground color shifting or vice versa.

Alternatively the color shifting ink is coated onto the surface oppositethe patterned aluminum so that one views the security device from thepatterned aluminum side. In this instance, the color shifting ink showsthrough the openings of the patterned aluminum. As in embodimentsdescribed heretofore, the patterned aluminum can have an additionalprotective layer placed upon it, such as a scratch resistant lacquer oris laminated to a thin PET sheet typically having a thickness of 0.5 milor less. FIGS. 9 and 10 exemplify these structures.

In FIG. 9 a plastic web 90 has on an upper side thereof, an opticallyvariable structure 92, for example in the form of optically variableink, color shifting ink, optically variable pigment or a thin filmFabry-Perot cavity structure. On a lower side of the web, a patternedlayer of aluminum 94 is shown. In FIG. 10 a color shifting ink layer 106is disposed upon a plastic substrate 100 and a patterned layer ofaluminum 104 is deposited on the lower side of the substrate 100 havinga protective lacquer coating 106 over it.

In one embodiment, the reflective layer is a layer of opaque aluminum,so that the window portions of the patterned layer appear reflective.The backside reflector does not typically form an OV structure with thefront side MgF2-Cr layers because the intervening PET web is relativelythick for use as a spacer in a Fabry-Perot structure in the visiblerange of light. The window portions appear mirrored, while thefront-side F-P structures provide an OVD as is shown in FIG. 3.

Alternatively, the optical structure on the backside of the web is anoptical interference structure, such as a thin-film absorber layer onthe PET web, a spacer layer over the absorber layer, and a reflectivelayer over the absorber layer, thus creating a second F-P structure inaddition to the F-P structures on the frontside of the web as is shownin FIG. 11. A reflective backside layer 112 a is particularly desirablefor security threads with OVDs because the mirror-like backgroundprovides a good visual reference to the color change of the frontsideOVD. This reflective backside layer also serves as a layer in theoptically variable structure defined by the two adjacent layers; a Crlayer 115 a, a dielectric layer 111 a. FIG. 11 also shows a plasticsubstrate 110 having a patterned aluminum layer 112, followed by adielectric layer 1 having a layer of Cr 115 over top.

Alternatively, a layer of color shifting ink may be applied to thebackside of the web. The application of color shifting ink to thebackside of the web enables an optically variable (OV) effect when thestructure is viewed from either side. When viewed from the backside, theOV effect of the color shifting ink is observed. When viewed from thefront side, the OV effect of the color-shifting ink structures formedwith the patterned Al layer is observed, in addition to the OV effect ofthe color shifting ink.

Turning now to FIG. 12, a PET web 120 is shown with patterned aluminum112 on the front side and an optical structure on the backside. In oneembodiment the optical structure is an organic layer 128 containingAnti-Stokes material in the form of powder. The Anti-Stokes layerfluoresces at a shorter wavelength when illuminated at a longerwavelength. Many suitable materials exist, and are typically applied tothe backside of web as very fine particles in a carrier. PowderedAnti-Stokes material is available from STAR DUST TECHNOLOGIES. Theparticles are generally light-colored, such as a cream or a light tancolor, and fluoresce in a color, such as blue, green, yellow, or orangewhen irradiated with near IR light, which is outside the visible range.Thus, when the window portions are irradiated with the near IR light,the window portions fluoresce in a visible color. This can be used as acovert security feature because the Anti-Stokes coating is not easilyseen by casual observation.

Alternatively, shown in FIG. 13, a patterned optically variable foilcomprising a patterned layer of aluminum 132, a dielectric layer 131,and a Cr layer 135 may be formed on one side of the PET and a printedlayer of regular ink 136 is printed on the second side of the PETsubstrate 130. The printed ink 136 is either black in color or is acomplimentary color to the normal (90 degree) color of the colorshifting foil. In this case, the black or complimentary color showsthrough the windows of the patterned OVD or in the reversed image, theblack or complimentary color layer acts as a background for the colorshifting patterned text. For instance, a green to blue color shiftingfoil would have a magenta printed ink on the second side of the PET; forexample, green and magenta are complimentary colors. In both instances,the contrast between the printed ink layer and the color shifting foilallows easy viewing in reflected light. A matching color could also beprinted so that the reflected colors of the text only show up at otherangles than the one at normal.

In an embodiment shown in FIG. 3 b, the plastic web a plastic film 30has an embossed surface coated with an aluminum layer 32 on the sideopposite the patterned coating 34 so that the holographic imagery ordiffractive imagery shows through the holes 36.

FIG. 14 shows an OVD (optically variable device) formed on a web 140wherein a reflective layer 142, such as an Al layer is deposited, and anoverlying spacer layer, such as an MgF2 layer 144, is patterned, withthe oil-“inhibiting” technique. An absorber layer 146, such as a thinlayer of Cr, is deposited over the underlying layers to form OVDs wherethe reflective, spacer, and absorber layers form an F-P structure. Aswith the OVD shown in FIG. 3 wherein the optical structure on thebackside is a reflective layer, the embodiment shown in FIG. 14 may havehighly reflective regions proximate to the OVD structures that providevisual references when observing the OVD. In another embodiment, moredielectric layer material can be deposited onto the patterned dielectricso that a color-shifting pattern is formed on top of a differentcolor-shifting pattern, once the final absorber layer has beendeposited.

FIG. 15 shows an OVD formed on a web 150 with reflective and spacerlayers 152 and 154 respectively formed on the web and an overlyingpatterned absorber layer 156. The absorber layer is patterned using theoil-ablation techniques. An OVD is formed where the absorber materialremains.

FIG. 16 shows a simplified coating machine according to an embodiment ofthe invention. The coater includes an unwind roller 160 a, a wind roller160 b, a printing head 162, an evaporation boat 164 and glow dischargeelements 166 a and 166 b. In operation, the printing head 162 receivesthe “inhibiting oil” oil from the pick-up roller 168 and applies the oilto a PET web 170.

The oil has the property that it does not readily evaporate in thevacuum of the roll coater, but readily evaporates when subjected to theheat of condensation of the evaporating material. In addition, the oilhas the property that is does not spread on the surface of the plasticweb i.e. there is little, if any, dot gain.

The oil must stick to the substrate but not spread beyond the image areathat is printed. Ideally, it should have an interaction with the plasticweb but not spread. If it spreads beyond the image of the imprintingcylinder, the image will not be faithfully reproduced. If the graphicimages are in pixel form, it is important that pixels aredistinguishable from one another and do not have edges or portions thatrun into each other. Such an unwanted increase in pixel size is known asdot gain. For very thin layers of oil it likely does not make anydifference whether it is wetting oil or non-wetting oil. However, it ispreferable that oil not bead up.-If the oil was thick and non-wetting,it would simply bead up and run off of the web and not maintain theprinted image. The spreading of one material on another is determined bythe respective surface energies, γA, γB and γAB, where γA is the surfacetension (ie. surface energy) of the plastic web, γB is the surfacetension of the oil and γAB is the interfacial surface tension. Spreadingis determined by the equation S_(L/s)=γA−γB−γAB, where S_(L/S) is thespreading coefficient. If S_(L/S) is positive, spreading will occur. Inother words, γA is larger than the sum of γB and γAB which means (γBplus γAB) has a lower surface energy than γA. Thus, to minimize energy,spreading will occur. Therefore, S_(L/S) should be negative so thatspreading of the oil on the plastic substrate does not occur. Asmentioned above, spreading would be detrimental since the dot gain wouldreduce the resolution from the original print image on the patternedimaging roller. The oil also has a low vapor pressure so that it doesnot evaporate after printing the image onto the web. FOMBLIN or Krytoxoils meet the spreading criteria; however, other low vapor pressure oilsmay be used depending on the substrate being used.

In operation, the web is advanced to the evaporation boat, whichdeposits Al or other material on the surface of the web. Processconditions are controlled so that the heat of condensation of the Alvaporizes the oil where it underlies the Al, removing the Al from theregions that previously had oil. An alternate explanation is that theoil prevents nucleation of the depositing aluminum, i.e. the aluminumdoes not stick and re-evaporates into the chamber. However, even thoughthe process occurs under vacuum, it was discovered that either productsof oil breakdown and/or residual oil (generally “oil residue”) may bepresent on some portions of the web, including the backside of the webafter vacuum deposition. Such residue can degrade the opticalperformance of OVDs subsequently formed on the web and preventsacceptable adhesion of subsequent then film layers. It is very difficultto even detect the presence of such residue, absent forming an OVD andevaluating its optical performance. Ghosting, smearing, and otherundesirable effects were observed when thin-film OVD structures weredeposited on webs with patterned Al without some cleaning of thisresidual oil. The Al was deposited and patterned in one vacuum coater,and the OVDs were deposited in another vacuum coater. It was found thatoil residue removal can be performed in the Al coater after patterning,in the OVD coater before deposition, or that a single vacuum roll coatercan be used to deposit and pattern the Al and to deposit the OVD layers,with an intervening cleaning.

A glow discharge cleaning technique was used to successfully remove oilresidue. Several types of glow discharge cleaning techniques wereevaluated, as well as other cleaning techniques. A glow dischargecleaning technique using argon gas was tried, but did not adequatelyclean the residue of the oil used in the patterning process. IR heaterswere used before coating, but oil transfer still occurred, presumablybecause oil transfer between layers of the web occurred. For example,oil residue on the frontside of the web will be transferred to thebackside of the web on the wind roll. In an in-line process that coatsthe OVD before layers of the web are wound together, frontside cleaningof residue may be sufficient. However, with the oil used in thisexample, it is believed that some amount of cross-contamination occurredduring the patterning process, and cleaning both the fontside and thebackside was performed, as shown in FIG. 17.

Many additional features of the coater are omitted, such as tensioningrollers and chamber dividers. Similarly, the web may be held against adrum during the Al deposition/patterning.

FIG. 17 shows a simplified depositing apparatus for cleaning a patternedweb. The patterned web 170 comes off the unwind roller 170b and along aseries of tension rollers. The backside of the web is cleaned at a firststation with a first glow bar 171, shown as being unshrouded. Thefrontside of the web is cleaned at a second station 172 further alongthe web with a glowbar that is shrouded. The shroud is optionallyomitted if the glow discharge does not affect other system components.Similarly, cleaning the backside of the web might not be necessary inall embodiments. Oxygen was provided to the glowbar shroud, but could beprovided at other locations of the chamber. The oxygen provided at theshroud diffused to the region of the first glowbar to create thecleaning discharge. A chamber divider 179 keeps material from the OVsource(s) from depositing on the tension rollers and other systemcomponents. The web is tensioned against a coating drum 178 duringdeposition of the OV layers. A glow discharge using 02 as the precursorwas found to work well to clean the web of oil residue. Other precursorsmay be preferred for other oils or liquids, or even for this type of oilresidue. In a particular embodiment, a web of PET about 8.5 inches widewas transported through a glow discharge cleaning stage at a speed of0.5 meters/second. The total current to the backside glowbar and thefrontside glowbar was 100 mA and the glowbars were operated at 2,200Volts. Pure O₂ was provided to the shroud of the frontside glowbar tocreate a chamber pressure of 5×10⁻³ Torr to remove the residual oil fromboth sides of the patterned web. An OVD that was formed on the patternedweb after cleaning in this fashion showed good optical characteristics,suitable for use in commercial applications.

FURTHER DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The invention is generally directed toward methods and apparatus formanufacturing imaged foils, as well as security articles includingoptically variable foils and security labels. In a particularembodiment, an optically variable foil is produced using an all vacuumin-line process.

A substrate consisting of 0.25 to 5 mil PET, preferably 0.5-1.0 mil (1mil=25.4 microns) is first patterned on the web, width 2″-60″ or more,with a positive or negative image with perfluorinated polyether asdescribed in U.S. publication “Proceedings of the FourteenthInternational Conference on Vacuum Web Coating, Oct. 25-27, 2000 byAerre Machine. A printing station similar to that described in U.S. Pat.No. 4,749,591 was used and is incorporated herein as a reference.However, the printing method is not necessarily limited to thatdescribed in U.S. Pat. No. 4,749,591. For example, other printingtechniques may be used include ink-jet printing, flexographic printing,gravure printing or lithographic printing, or even dot matrix printing.In the case of inkjet printing, it is possible to change the pattern orimagery without breaking vacuum. Advantageously, this allows customizedpatterns to be created on the plastic roll without the need for a rollon a printing station that requires a pre-imaged print roller. Suchpatterning with an ink jet process would allow sequential numbering ofsecurity labels and other security devices, which is highlyadvantageous. Multi-layer patterning using a single patterning roll orusing different patterns in subsequent thin film layers using the inkjet printing process may be used to create complex graphics or evenmicro-electronics in the security thread. In particular, a securitythread with an optically variable feature may be combined with a hiddenmagnetic bar code in either one dimension, as in a standard bar code, orin a two dimensions, as in a 2D bar code format. The magnetic layer ishidden between other layers of the thin film design, as for examplebehind a reflective layer or sandwiched between two highly reflectivealuminum layers as shown in FIGS. 18 and 19.

To insure good image fidelity, the printing station should be situatedon the cooled drum immediately before the deposition of the first layer,typically aluminum, but may be any material with a heat of condensationsufficient to vaporize the imaged oil. Other substrates such aspolyimide, polyhexadiene, polypropylene, polyethylene, polystyrene,polycarbonate triacetate, biacetate, and polynathphanate (PEN) may beused instead of the polyethylene terephthalate (PET). With othersubstrates and other surfaces, patterning oils based on the fundamentalsurface energies encountered and required low vapor pressures asdescribed above would be used.

Depositing the first layer that is to be imaged is the next process stepto be performed. In the case of an OVD security image that is opticallyvariable, either the reflector layer or the absorber layer may beimaged. In general, any layer may be imaged as long as the oilpatterning process produces a discreet oil image. For example, theimaging may occur in the dielectric layer by placing the oil image onthe prior-deposited layer. In other words, the reflector layer or theabsorber layer may be the layer upon which the oil image is placed.Deposition of a metal layer onto the patterned oil results in explosiveevolution of liquid oil into gas, causing the depositing layer to beablated away. There may be some residual oil (several monolayers of oil)remaining in the patterned area which must be removed in order to thisimaged oil from transferring further down the web which would in turncause ghosting (another image pattern) of any subsequent depositionlayers.

In a typical case, the aluminum layer or the chromium layer is imagedand then the rest of the design is added to complete a Fabry-Perotstructure, i.e. Al (opaque, patterned)/MgF₂ 1 QW@400 nm to 8QW@700 nm/Cr30% T. The low index MgF₂ layer may be substituted by any dielectricmaterial that is highly transmissive in the visible. High indexdielectric materials will result in an optically variable foil that isless shifting in color than one that has a large optical shift where alow index dielectric material is used. A partially aluminum layer thathas a thickness below the opaque point, for example, thickness in therange 200-800 nm, will give a color shifting film that is partiallytransparent so that information may be read through the optical stackfrom the paper, or that is printed on the PET. All color shifts movefrom long to shorter wavelengths, i.e. from red-to-blue.

To remove residual oil in the patterned area, an oxygen glow situatedright after the deposition source on the cooled drum is used. Glows mayalso be used on the backside of the web before and after the depositionarea. In this instance, the 02 glow will remove any oil from thebackside of the web that might have transferred during the “inkingprocess” or flash over during the deposition process that could end upon the back side during wind-up. Typically, the oxygen glow is run at2,200 volts at 100 ma for a 12″ wide glow system. Residual oil may bedetected on the final coated web by noting the variation in color, forexample blotches or ghost like images of the original images.

Resolution of the image using the flexographic printing process hasshown to be as low as 20 microns although 70 microns is nominal. This isshown in FIG. 20. Besides text, a graphic image can be made byprocessing a scanned image using a FloydSteinburg technique as found inthe software program COREL DRAW™. This program converts the image intosquare pixels, which have good black (image) and white (no image)contrast. After coating, the web is slit into ribbons between 1 and 5 mmwide wherein the text is generally situated in the center of the ribbon.FIG. 21 show the optically variable thread of this invention windowedinto a bank note and FIG. 22 shows the text symbols when viewed intransmission.

Instead of a ribbon, the device may function as a security stick-onlabel by applying adhesive 232 to one side of the PET and laminating toa release layer 234 supported by paper carrier 236 and die cutting thelabel 230 as is shown in FIG. 23. The adhesive can be a solvent based ora water-based adhesive. A suitable adhesive is an acrylic adhesive,among many others. Instead of a label, the product could be made into ahot-stamp security product by inserting a release layer between thecarrier web and the vacuum deposited layer.

Of course numerous other embodiments may be envisaged without departingfrom the spirit and scope of the invention.

1. A security thread for embedding within or upon a sheet or document,comprising: an elongate substrate having a first side and a second side;an optically variable structure deposited on one of the first and secondsides of the elongate substrate, wherein the optically variablestructure comprises a thin film interference structure, which appears asa plurality of separated interference filters arranged side-by-side andspaced from one another having visible color shifting properties in theform of a visible pattern of visually separated distinguishable indiciaagainst one of a foreground and background of a different color.
 2. Asecurity thread as defined in claim 1, wherein the visible pattern ismade by the steps of: a) coating the substrate with a first material inthe form of the pattern or its inverse; b) applying a layer of a secondmaterial over the first material and portions of the substrate absentsaid first material adjacent to where the first material is coated; c)removing the first material and the layer of the second materialcovering it, leaving the second material in regions where the substratewas not coated by the first material and, d) coating a third and afourth layer over the second material to form a plurality of Fabry-Perotfilters in discrete regions.
 3. A security thread as defined in claim 1,wherein the first side of the elongate substrate has deposited thereon afirst layer of reflective material comprising a plurality ofside-by-side, spaced apart reflective regions, at least some of thespaced-apart reflective regions forming a first layer of theinterference structure comprising a plurality of n-layered Fabry-Perotcavities.
 4. A security thread as defined in claim 1 wherein a secondoptical structure is deposited on the second side of the elongatesubstrate and wherein a pattern comprising at least a patterned layer ispresent on the first side of the elongate substrate.
 5. A securitythread as defined in claim 4 wherein the second optical structurecomprises an organic layer containing an anti-Stokes pigment.
 6. Asecurity thread as defined in claim 1, further comprising a magneticlayer within or behind the interference filters.
 7. A security thread asdefined in claim 1, further comprising a Fabry-Perot structure depositedon an opposite side of the elongate substrate to the separatedinterference filters and wherein one of the Fabry-Perot structure andthe interference filters have windows that allow the other to be seentherethrough.
 8. A security thread as defined in claim 4 wherein thesecond optical structure comprises a color shifting ink.
 9. A securitythread as defined in claim 1 further comprising a magnetic layerdeposited between at least some of the side-by-side spaced apartinterference filters.
 10. A security thread as defined in claim 1,further comprising a magnetic layer disposed upon a reflective layer orcovered on at least one side by a reflective layer common to theplurality of separated interference filters.
 11. A security thread asdefined in claim 1 further comprising a magnetic layer disposed withinand or between or behind the interference filters, wherein the magneticlayer contains machine readable information for verifying theauthenticity of the thread or sheet.
 12. A security thread as defined inclaim 1, wherein the security thread is disposed within or upon thesheet, and wherein the sheet forms one of an imaged security label,currency or passport security device.
 13. The security thread as definedin claim 1, wherein the thin film interference filter includes a colorshifting ink.
 14. The security thread as defined in claim 1 furthercomprising color shifting ink.
 15. The security thread as defined inclaim 1, wherein the thin film interference filter includes an opticallyvariable pigment.
 16. A method for forming an optically variable deviceupon a web, comprising the steps of: patterning a reflective layer on aweb having a first side and a second side using an oil-ablationtechnique; removing oil residue from the first side of the websubstrate; and depositing thin-film layers supported by the web to formthe optically variable device.
 17. A method as defined in claim 16,wherein the step of patterning the reflective layer is performed on thefirst side of the web, and wherein the thin-film layers are deposited onthe first side of the web over the patterned reflective layer.
 18. Themethod of claim 17, further comprising the step of removing oil residuefrom the second side of the web.
 19. The method of claim 16, furthercomprising the step of depositing an optical structure over thepatterned reflective layer, or behind the reflective layer on anopposite side of the web.
 20. The method of claim 16, wherein the stepof removing oil residue comprises using a glow discharge.
 21. The methodof claim 20, wherein the glow discharge is formed from a precursorcomprising oxygen.
 22. The method of claim 16, wherein the oil ablationtechnique includes the step of applying oil in a pattern with an ink jetdevice.
 23. A security thread as defined in claim 1 wherein the elongatesubstrate has an embossed diffractive pattern coated with a reflectivelayer on a side of the substrate opposite to the optically variablestructure, such that the embossed pattern is at least partially visiblefrom a side of the substrate having the interference structure.
 24. Asecurity thread as defined in claim 1 further comprising a magneticlayer disposed within and or between or behind the interference filters,wherein the magnetic layer has a thickness of less than about 5.0microns.